Variable speed drives utilizing chains and sprockets have been employed with bicycles for many years. The drawbacks of such systems are well known and are described in U.S. Pat. No. 4,030,373 to the present inventor. That patent discloses a variable ratio transmission for bicycles which includes a plurality of movable sheave segments, each sheave segment having a releasable, toothed retaining means which normally retains the sheave segment at a fixed radial position in a toothed track. That structure is, essentially, a variable diameter pulley or sheave whose diameter is adapted to be selectively adjusted by the rider. A flexible belt is wrapped around and engages different adjacent sheave segments to impart rotary motion to the drive mechanism. The relative position of each sheave segment in its toothed track is adjusted only when a sheave segment comes out of contact with the drive belt.
The mechanism described in the '373 patent for locking each sheave segment into place after adjustment contains relatively small and highly stressed parts requiring close manufacturing tolerances. The setting mechanism can be sensitive to both axial location and warpage. Locking surety also may degrade somewhat with wear.
In U.S. Pat. No. 4,503,676, also to the present inventor, a variable ratio drive mechanism is disclosed which also employs driving and driven sheaves, each of which is provided with a set of adjustable sheave segments. In that mechanism, individual sheave segments are one-piece, belt-loaded-locked units which engage saw-tooth shaped steps along associated trackways. The particular asymmetric geometry shown is such that the belt's force on each sheave segment applies an offset torque which forces the sheave segment's teeth into engagement with opposed saw tooth steps along one side of the track. When each sheave segment becomes free of the force of the belt, it can be engaged by a shifter which causes its teeth to move out of engagement with the track's steps. The sheave segment is then radially movable in either an outward or inward manner. In order to unlock the sheave segment's teeth from engagement, means are provided to cause a modest amount of rotation of a segment's teeth so that they can ratchet up or down relative to the track's steps. This design is somewhat less suitable for small sheave diameters than large diameters and for applications involving relatively resilient belts which are subjected to grossly fluctuating driving tensions. Furthermore, the design is adapted only to a single direction drive.
In U.S. Pat. No. 4,816,008, also to the present inventor, there is disclosed still another transmission of the type that includes sheave segments coupled together by a drive belt. That transmission employs a sheave segment locking mechanism which runs the full length of each disk track in the drive mechanism. The locking mechanism described therein is controlled by a fixed path cam whose action is related to the radial position of the sheave. More specifically, the locking mechanism is released and removed from interaction with an individual sheave segment by a cam means which is operative only when the sheave segment is out of contact with the drive belt. Under those circumstances, the sheave segment is free floating and can be either moved inwardly or outwardly by a shift mechanism. In this mechanism, positive and consistent lock-up is dependent upon light springs and free fitting, cooperating parts. Relatively close tolerances are required and lock-up surety may decrease with wear.
Yet another noteworthy prior art construction is disclosed in U.S. Pat. No. 4,832,660, also to the present inventor, which relates to an apparatus for positioning a bearing surface relative to a track. In one embodiment, that invention includes a rotatably mounted drive mechanism which is provided with a plurality of radially oriented tracks. The drive mechanism preferably comprises a pair of parallel drive disks with co-linear, radial tracks having tooth-like formations arranged therein. A movable sheave segment is mounted between each set of co-linear tracks. Each sheave segment comprises two pairs of engagement blocks and two pairs of springs, one pair for each track and a multi-element cam bearing means. Toothed means on the engagement blocks provide means for locking each sheave segment into place in the toothed track. Another broadly similar construction is disclosed in U.S. Pat. No. 4,820,244 to Lander. In both cases, engagement blocks and the multi-element cam bearing means are spring-connected.
In each disclosure, the bearing surface of each sheave segment is engaged by an endless belt when the drive mechanism traverses through a predetermined arc of rotation but is disengaged from the drive belt when outside the predetermined arc of rotation. The multi-element cam locking means associated with each sheave segment is forced by belt pressure to rigidly bias a sheave segment's engagement blocks against the tooth-like formations in the track. Spring means are provided to resiliently bias the engagement blocks into engagement with the track's tooth-like formations so that the sheave segments are lightly detented in place when out of engagement with the drive belt. In each case, detenting action acceptable for certain applications may be insufficient to prevent damaging radial fly-out under speeds routinely experienced in certain other applications.